Conventionally, there has been a phase modulation method in which information is superimposed on the phase difference between two optical pulses with a time difference, namely a signal optical pulse and a reference optical pulse. A phase modulation apparatus employing this phase modulation method has been used for quantum cryptography communication. When an electrical pulse is applied to an optical pulse input to a phase modulator provided in the phase modulation apparatus, the optical pulse is phase-modulated (see FIG. 9).
In the above conventional phase modulation apparatus, as can be seen in the configuration of the transmitting side apparatus in Patent Literature 1, an electrical pulse for phase modulation is applied only to a signal optical pulse, and a reference optical pulse passes through the phase modulator without being phase-modulated (see FIG. 10). Alternatively, as can be seen in the configuration of the receiving side apparatus in Patent Literature 1, a signal optical pulse passes through the phase modulator without being phase-modulated and an electrical pulse for phase modulation is applied only to a reference optical pulse.
Patent Literature 1: JP 2007-251678 A